Let’s look at what happens when formation pressure is greater than mud pressure, shall we?
When drilling penetrates a formation, that formation may contain fluids under pressure.
If the pressure from the mud column (hydrostatic pressure) is lower than the formation pressure, formation fluids can enter the wellbore.
This is called a kick.

Let me give a little guide before we continue, and that will be ‘What is hydrostatic pressure?’

Hydrostatic pressure is the pressure exerted by a column of fluid due to its weight.
Now, imagine diving into a swimming pool. The deeper you go, the more pressure you feel in your ears.
Why?
Because the water above you has weight.
That weight creates pressure.
That is hydrostatic pressure.

How Mud Prevents It?
Drilling fluid controls formation pressure through its density (mud weight).
Hydrostatic pressure depends on:
• Mud weight
• True vertical depth (TVD)
The deeper the well and the heavier the mud, the greater the hydrostatic pressure exerted.
By carefully adjusting mud weight, we maintain pressure balance and prevent influx (KICK).
 
Practical Reinforcement
If mud weight is too low → Kick risk
If mud weight is too high → Formation fracture risk

Food for thought:
Have you ever wondered how engineers determine the right mud weight for a specific formation?